A directional microphone apparatus that processes signals obtained from a plurality of non-directional microphone units to acquire directivity is known. As one method of this signal processing, there is a pressure-gradient directivity synthesis method. This synthesis method has an advantage in that the directivity can be formed even with the microphone units being arranged relatively in a small scale, whereas on the other hand has a defect in that the directivity is deteriorated when there are individual differences such as a level difference and a phase difference between the microphone units.
The level difference and the phase difference between the microphone units occur especially in a low band, due to an influence of air leakage and the like from a gap which is generated in a swaged section at a back side of the microphone unit due to a variation in mass-production or aging. Thus, the level difference and the phase difference still exist at a larger or smaller degree, even among the microphone units which quality is guaranteed by having undergone an inspection procedure at the time of shipment.
For example, Patent Literature 1 discloses a directional microphone apparatus that corrects only the level difference between two non-directional microphones by using levels of respective level of low band components of the two non-directional microphone units. FIG. 1 illustrates a configuration of the directional microphone apparatus disclosed in Patent Literature 1. FIG. 1A illustrates a case in which the level difference is corrected by a feedback, and FIG. 1B illustrates a case in which the level difference is corrected by a feedforward. Here, an explanation will be given using FIG. 1A.
In FIG. 1A, firstly, first and second non-directional microphones 11 and 12 pick up first and second signals. Next, level control circuit 19 performs level control on the second signal. Next, level control signal forming circuit 20 detects a level difference in the low band components between the first signal and the level-controlled second signal, and generates a level control signal corresponding to this level difference. Finally, level control circuit 19 controls by using the generated level control signal so as to remove the level difference between the first and second signals.
Further, for example, Patent Literature 2 discloses a directional microphone apparatus that plays learning signals from a speaker provided within the apparatus and performs calibration of microphone units. FIG. 2 illustrates a configuration of the directional microphone apparatus disclosed in Patent Literature 2.
In FIG. 2, firstly, signal processor 29 plays periodic noise signals that were set in advance, via amplifier 26 and from speakers 25 provided within detection areas of microphone units 21 to 24. Next, a digital FIR (Finite Impulse Response) filter in signal processor 29 performs filtering process on each signal picked up in microphone units 21 to 24. Finally, signal processor 29 is configured to evaluate a response from the digital FIR filter, adapt a characteristic of the digital FIR filter, and perform calibration of microphone units 21 to 24.
Further, for example, Patent Literature 3 discloses a directional microphone apparatus that adjusts frequency characteristic in a low band based on a sensitivity difference between two non-directional microphone units. FIG. 3 illustrates a configuration of the directional microphone apparatus disclosed in Patent Literature 3.
In FIG. 3, firstly, first and second non-directional microphones 11 and 12 pick up first and second signals. Next, first and second HPFs 30 and 31 perform high-pass filtering process on each of the first and second signals. Next, first and second BPFs 32 and 33 perform band-pass process that allows only frequency components in a particular band to pass on each of the first and second signals to which the high-pass filtering process has been performed. Next, sensitivity difference comparator 34 calculates which of the sensitivity differences of the first and second signals retaining only the frequency components in the particular band is larger. Finally, coefficient generating section 35 generates a coefficient of the HPF of the larger one of the first and second signal retaining only the frequency components in the particular band, based on the sensitivity difference. Here, as illustrated in FIG. 4, coefficient generating section 35 generates coefficients of a1 to an as coefficient a and coefficients of b1 to bn as coefficient b, according to the sensitivity differences d1 to dn (n being a positive number).